In order to quietly keep an interior of a vehicle from a vehicle exterior sound (including sounds generated from tires of running vehicle and engine thereof), porous material or composite material formed by adhering a sheet-like skin made of resin to fibrous material such as flexible polyurethane foam or felt is arranged on the back side of a member constructing a wall of vehicle interior (Patent Literatures 1 and 2).
Here, it is known that a sound absorbency of porous material becomes high in a high frequency region (See FIG. 6A), and a sound absorbency of membrane material becomes high in resonance frequency of membrane. In other words, in porous material a sound absorbency of a low frequency region is low, and in membrane material it is difficult to obtain a sound absorbency in a wide range of frequency region. When simply combining them, as shown in a model view of FIG. 6B, a sound absorption characteristic of a membrane vibration type is dominant, and thereby combination effect with a sound absorption characteristic of a porous type shown in FIG. 6A can not be expected.
Also, for example, a sound absorption characteristic of a frequency region of a motor is high in a portion near the motor, and a sound absorption characteristic of a frequency region of tire noise is high near tire. Furthermore, in a portion in which wind noise is likely to occur, a sound absorption characteristic according to the wind noise is high. For this reason, a request of soundproof material for vehicle having a partially different sound absorption characteristic is expected.
Patent Literature 1: JP 2006-240207 A
Patent Literature 2: JP 2005-120247 A